It is known that ultrasonic instruments can be useful in puncturing central veins and arteries. This more particularly applies in the case of image-forming ultrasonic instruments, on whose screens can not only be seen a sectional image of the investigated body region but also the puncturing needle, which can considerably facilitate puncturing. Such a method is e.g. described in the New England Journal of Medicine, 324:566, February 1991. However, it is difficult to sterilize ultrasonic probes, so that prior to use they are packed in a sterile, tubular cover, as is e.g. described in EP-A1-477,581. Laterally it is possible to fix to the ultrasonic probe and over the sterile cover a sterile coupling piece, which guides the puncturing needle in the ultrasonic sectional plane, such that said needle is visible on the screen (as described e.g. in U.S. Pat. No. 4,898,178). However, such image-forming instruments are not sterilizable and are so large that it is scarcely possible to bring about the sterile packing thereof and consequently they need a large setting-down surface outside the sterile operating field. An at least two meter long electric cable then connects the ultrasonic probe to the electronic casing, which is disadvantageous in the confined space conditions of intensive care units or catheterization units.
It is also known to use for the same purpose Ultrasonic-Doppler, which only reproduce acoustic signals and can therefore be much smaller and cheaper than the image-forming instruments. In most cases of such Doppler units the puncturing needle is virtually identical with the ultrasonic field axis. Therefore the piezocrystals of the ultrasonic probe and the puncturing needle are so spatially close together, that the ultrasonic probe is contaminated by the patient's blood. This, in particular applies to instruments, in which the piezocrystals are located within the needle or syringe, as described in U.S. Pat. Nos. 3,556,079, 5,131,394 and EP-A-260,953. The same contamination takes place if the ultrasonic probe is placed around the needle, as e.g., in CH-A-501,410. The contaminated ultrasonic probes can be gas-sterilized, but this process is time-consuming and also significantly reduces the life of the expensive ultrasonic probes. It is also expensive to dispose of the ultrasonic probe after a single use, as is recommended by certain manufacturers.
In EP-A1-0'516'582 a device is described, with which a non-sterile ultrasonic probe can be used, as it cannot contaminate neither the puncturing needle, nor the surgeon's gloves, nor the patient. A sterile coupling element, is planted on the non-sterile ultrasonic probe. In this coupling element the ultrasonic field is diverted by means of a mirror, such that the puncturing needle, guided by the coupling element, is directed towards the patient's skin parallel to and in the center of the ultrasonic field. The distance between the puncturing needle and the ultrasonic probe corresponds to at least half the diameter of the ultrasonic probe, such that the puncturing needle is unable to be contaminated. The coupling element must be manufactured to a high level of precision, because the ultrasonic probe, the mirror and the needle must be precisely positioned in relation to one another. Additionally the coupling element must be filled without gap with ultrasonic conducting gel and sealed with a puncturable elastomer membrane to prevent the gel from flowing out. Thus the coupling element, which is intended for single use, is relatively expensive.
Most known devices for sterile packing of ultrasonic units imply the collaboration of a sterile and a non-sterile person, as described in EP-A1-0'477'581 and EP-A1-0'516'582. According to EP-A1-0'540'461 a non-sterile person can hand the probe to a sterile person in the usual way. In this case however the two electric cables and the two electronic plugs that connect the ultrasonic probe with the electronics are part of the single use part of the whole instrument, which therefore makes this single-use part expensive.